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<a name="C_002b_002b-Attributes"></a>
<div class="header">
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<hr>
<a name="C_002b_002b_002dSpecific-Variable_002c-Function_002c-and-Type-Attributes"></a>
<h3 class="section">7.7 C++-Specific Variable, Function, and Type Attributes</h3>

<p>Some attributes only make sense for C++ programs.
</p>
<dl compact="compact">
<dd><a name="index-abi_005ftag-function-attribute"></a>
<a name="index-abi_005ftag-variable-attribute"></a>
<a name="index-abi_005ftag-type-attribute"></a>
</dd>
<dt><code>abi_tag (&quot;<var>tag</var>&quot;, ...)</code></dt>
<dd><p>The <code>abi_tag</code> attribute can be applied to a function, variable, or class
declaration.  It modifies the mangled name of the entity to
incorporate the tag name, in order to distinguish the function or
class from an earlier version with a different ABI; perhaps the class
has changed size, or the function has a different return type that is
not encoded in the mangled name.
</p>
<p>The attribute can also be applied to an inline namespace, but does not
affect the mangled name of the namespace; in this case it is only used
for <samp>-Wabi-tag</samp> warnings and automatic tagging of functions and
variables.  Tagging inline namespaces is generally preferable to
tagging individual declarations, but the latter is sometimes
necessary, such as when only certain members of a class need to be
tagged.
</p>
<p>The argument can be a list of strings of arbitrary length.  The
strings are sorted on output, so the order of the list is
unimportant.
</p>
<p>A redeclaration of an entity must not add new ABI tags,
since doing so would change the mangled name.
</p>
<p>The ABI tags apply to a name, so all instantiations and
specializations of a template have the same tags.  The attribute will
be ignored if applied to an explicit specialization or instantiation.
</p>
<p>The <samp>-Wabi-tag</samp> flag enables a warning about a class which does
not have all the ABI tags used by its subobjects and virtual functions; for users with code
that needs to coexist with an earlier ABI, using this option can help
to find all affected types that need to be tagged.
</p>
<p>When a type involving an ABI tag is used as the type of a variable or
return type of a function where that tag is not already present in the
signature of the function, the tag is automatically applied to the
variable or function.  <samp>-Wabi-tag</samp> also warns about this
situation; this warning can be avoided by explicitly tagging the
variable or function or moving it into a tagged inline namespace.
</p>
<a name="index-init_005fpriority-variable-attribute"></a>
</dd>
<dt><code>init_priority (<var>priority</var>)</code></dt>
<dd>
<p>In Standard C++, objects defined at namespace scope are guaranteed to be
initialized in an order in strict accordance with that of their definitions
<em>in a given translation unit</em>.  No guarantee is made for initializations
across translation units.  However, GNU C++ allows users to control the
order of initialization of objects defined at namespace scope with the
<code>init_priority</code> attribute by specifying a relative <var>priority</var>,
a constant integral expression currently bounded between 101 and 65535
inclusive.  Lower numbers indicate a higher priority.
</p>
<p>In the following example, <code>A</code> would normally be created before
<code>B</code>, but the <code>init_priority</code> attribute reverses that order:
</p>
<div class="smallexample">
<pre class="smallexample">Some_Class  A  __attribute__ ((init_priority (2000)));
Some_Class  B  __attribute__ ((init_priority (543)));
</pre></div>

<p>Note that the particular values of <var>priority</var> do not matter; only their
relative ordering.
</p>
<a name="index-warn_005funused-type-attribute"></a>
</dd>
<dt><code>warn_unused</code></dt>
<dd>
<p>For C++ types with non-trivial constructors and/or destructors it is
impossible for the compiler to determine whether a variable of this
type is truly unused if it is not referenced. This type attribute
informs the compiler that variables of this type should be warned
about if they appear to be unused, just like variables of fundamental
types.
</p>
<p>This attribute is appropriate for types which just represent a value,
such as <code>std::string</code>; it is not appropriate for types which
control a resource, such as <code>std::lock_guard</code>.
</p>
<p>This attribute is also accepted in C, but it is unnecessary because C
does not have constructors or destructors.
</p>
</dd>
</dl>

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